Substitution Reactions of Benzene and Other Aromatic Compounds

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The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).

Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions). Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution. The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column.

Reaction Type	Typical Equation				Electrophile E⁽⁺⁾
Halogenation:	C₆H₆	+ Cl₂ & heat FeCl₃ catalyst	——>	C₆H₅Cl + HCl Chlorobenzene	Cl⁽⁺⁾ or Br⁽⁺⁾
Nitration:	C₆H₆	+ HNO₃ & heat H₂SO₄ catalyst	——>	C₆H₅NO₂ + H₂O Nitrobenzene	NO₂⁽⁺⁾
Sulfonation:	C₆H₆	+ H₂SO₄ + SO₃ & heat	——>	C₆H₅SO₃H + H₂O Benzenesulfonic acid	SO₃H⁽⁺⁾
Alkylation: Friedel-Crafts	C₆H₆	+ R-Cl & heat AlCl₃ catalyst	——>	C₆H₅-R + HCl An Arene	R⁽⁺⁾
Acylation: Friedel-Crafts	C₆H₆	+ RCOCl & heat AlCl₃ catalyst	——>	C₆H₅COR + HCl An Aryl Ketone	RCO⁽⁺⁾

Contributors

William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry